According to Aristostle what is the "best way of life"

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Book 4 goes on and on about political philosophy as related to models by which one can judge other regimes and see which regime is the best possible in a given situation. So, the answer to your question must fall within this paradigm. Aristotle articulates "the best way of life" in other sections but if limited to book 4 you might consider the following,

An important point which Aristotle brings out toward the end of the book of the idea of the "middling element" in society. The middling element is basically what in modern terms would be considered the middle class. Aristotle praise of the middling element is logical, in that he believes virtue to be mean between two extremes of vice. A large middle class is absolutely essential for a stable and well-run government because the middle class do not covet rule, are not envious, foster friendship because of their similarity, and can act as neutral arbitrators between the rich and the poor. Hemmed in between people above it which it dislikes and people below it which it fears, the middling element is more likely to listen to reason and to help maintain stability in the regime.